1. Field of the Invention
The present invention relates to an apparatus which compensates for dispersion in an optical fiber transmission line. More specifically, the present invention relates to an apparatus which includes a fixed dispersion compensator for coarse compensation and a variable dispersion compensator for fine compensation.
2. Description of the Related Art
Optical transmission systems using fiber optical transmission lines are being used to transmit relatively large amounts of information. For example, optical transmission systems at 10 Gb/s are now in practical use. However, as users require larger amounts of information to be rapidly transmitted, a further increase in the capacity of optical transmission systems is required.
However, in an optical transmission system, as transmission speed increases, the transmission distance is severely limited because of wavelength degradation caused by group-velocity dispersion (GVD) in an optical fiber. Furthermore, when the transmitting optical power is increased to maintain the required transmit/receive level difference, the effect of self-phase modulation (SPM), a fiber nonlinear effect, increases. This increase in SPM further complicates the waveform degradation through interaction with the group-velocity dispersion (SPM-GVD effect).
As an example, consider an optical transmission system having transmission lines which use single-mode fibers (SMFs) having a zero dispersion wavelength in the 1.3 .mu.m region. This type of SMF is the most common type of fiber currently being used in existing fiber transmission lines. In such an optical transmission system, the chromatic dispersion value at a signal light wavelength of 1.55 .mu.m (where transmission loss of silica-based fiber is the lowest) is as large as about +18 ps/nm/km. As a result, dispersion compensation techniques are required for 10 Gb/s and higher-speed transmission systems where a relatively small amount of dispersion can be tolerated.
For example, according to an experiment with a 40 Gb/s SMF transmission over a distance of 50 km (see G. Ishikawa et al., ECOC' 96 ThC.3.3 for the transmitter/receiver configuration), the dispersion compensation tolerance when the power penalty is 1 dB or less is extremely small, i.e., 30 ps/nm. Therefore, in a 40 Gb/s SMF transmission system, highly precise dispersion compensation must be performed for each repeater section in the system.
Transmission lines using 1.55 .mu.m band dispersion-shifted fibers (DSFs) have been installed in recent years for long-distance transmission at 10 Gb/s. However, because of slight variations in fiber core diameter introduced when drawing fibers in the optical fiber manufacturing process, the zero dispersion wavelength .lambda..sub.0 varies from one repeater section to another. Further, even within the same repeater section, .lambda..sub.0 varies along the length of fiber. In addition, a transmission cable is usually constructed by joining together multi-core cable segments each a few kilometers long. That is, there is no continuity in .lambda..sub.0 between adjacent segments, so .lambda..sub.0 has a random distribution profile. As a result, a variation of .+-.10 nm can occur within one repeater section, and the state of the variation differs from one repeater section to another. Strict dispersion compensation is therefore necessary in a 40 Gb/s long-distance DSF transmission system.
In optical transmission systems with transmission speeds up to 10 Gb/s, since the dispersion tolerance is relatively wide, system design is possible that allows the common use of a dispersion compensator having a predefined dispersion value, such as a dispersion-compensating fiber (DCF) or a fiber grating, over a transmission distance of 20 to 40 km. However, when the dispersion compensation tolerance is extremely small, as in 40 Gb/s systems, the amount of dispersion compensation must be optimized for each repeater section. The only ways to achieve such dispersion compensation, at the present time, are:
(i) To fabricate a dispersion compensator that matches the actually measured value of the chromatic dispersion of the transmission line; or PA1 (ii) To prepare "units" of DCFs or fiber gratings whose dispersion values are different by small amounts, and change the combination of units to be inserted according to the actually measured value of the chromatic dispersion, similar to the manner in which an object is weighed on a balance.
In the case of (ii) , however, if multiple units are connected, the apparatus size increases. Moreover, if the units are joined together by connectors, the total insertion loss increases. If the value of the chromatic dispersion is unknown, optimization can be achieved by inserting and removing units, but this leads to an enormous increase in man-hours and also a waste of units.
Furthermore, neither (i) nor (ii) can be applied to cases where the value of the chromatic dispersion changes over time due to transmission line (waveguide) temperature, external pressures, or vibrations.
Therefore, for an ultra high-speed system such as a 40 Gb/s system, the development of a "variable dispersion compensator" capable of varying the amount of dispersion with a single device is essential. As a variable dispersion compensator, there has been proposed a Planar Lightwave Circuit (PLC) dispersion compensator capable of varying its dispersion amount from -383 ps/nm to +615 ps/nm (for example, see K. Takiguchi et al., ECOC' 93 ThC 12.9, which is incorporated herein by reference). However, a variable dispersion compensator with a variable range of -383 ps/nm to +615 ps/nm can only support transmission distances up to about 20 km in the case of an SMF having a chromatic dispersion value of +18 ps/nm/km. Also, commercial implementation is difficult in terms of manufacturing as well as from the viewpoint of controllability.
There has also been proposed a method in which, in a fiber grating dispersion compensator, a temperature gradient is provided using a Peltier element, or an external stress is applied to the fiber grating itself using piezoelectric elements, to provide the capability to vary the amount of dispersion compensation (for example, see R. I. Raming and N. N. Zervas, ECOC' 96 Short courses, which is incorporated herein by reference). This method, however, involves problems such as complex control and narrow bandwidth (see, for example, M. Kato and Y. Miyajima, OECC' 97 9D1-2, which is incorporated herein by reference) , and is not yet ready for commercial implementation.
Therefore, as described above, known or proposed variable dispersion compensators are difficult to design, manufacture, and control, and lack practicability, since they are designed to combine (i) a large dispersion amount with (ii) a large variable range in a single dispersion compensator.